In microlithography for integrated circuits, there is the problem of configuring the imaging of adjacent openings in the photoresist such that the distance between the structures produced becomes minimal. Every reduction of the distance leads to a reduction of the chip area, thereby making it possible to reduce the costs per chip.
To date, use has usually been made of binary masks (BIM) or halftone masks HTPSM with lithographically more favorable properties, mainly for projection exposure into a wafer coated with positive resist.
In the foreseeable future, with shorter wavelengths in the range of 1 to 100 nm, in particular 10–15 nm, EUV reflection masks (EUV=extreme ultraviolet region) will be used more and more frequently.
Such EUV reflection masks are based on the principle of distributed Bragg reflection, according to which EUV light is reflected at periodically recurring interfaces (multilayers) and interferes constructively in the process. On account of the constructive interference of a plurality of multilayers (typically 40 Mo/Si double layers), the reflectivity is increased to more than 70% in the case of non-glancing incidence.
In order to produce a lithographic imaging, it is necessary to eliminate or reduce the reflectivity of the multilayers locally at the locations at which no imaging is intended to be effected.
In a demagnifying optical arrangement, the mask image is projected onto a resist-coated wafer using such an EUV reflection mask.
FIGS. 3a, b show the layout of a known photomask in two successive fabrication stages.
In FIG. 3, reference symbol 1 designates a glass substrate with a low thermal expansion coefficient. This glass substrate 1 is coated with the layers explained in more detail below in order to form the EUV reflection mask.
The layer 5 designates an optional buffer layer e.g. made of silicon, which serves for smoothing possible unevennesses of the substrate surface of the glass substrate 1.
The multilayer coating 10, which serves to reflect the incident light, is then provided on the buffer layer 5. In the example considered, what is involved is an alternate sequence of silicon and molybdenum layers having a thickness of the order of magnitude of 5 to 8 nanometers corresponding to a wavelength used in the range of 10 to 16 nanometers.
A covering layer 15 made of silicon typically having a thickness of 11 nm is deposited above the multilayer coating 10, which covering layer serves as it were as a chemical protection layer.
An upper buffer layer 20, e.g. made of silicon oxide, is then deposited over the covering layer 15, which buffer layer is intended to afford protection of the underlying layers during the patterning of the overlying absorber layer 25 made of chromium. This patterning is effected for example by means of a photolithographic ion etching process and leads to the process state shown in FIG. 3a. 
With reference to FIG. 3b, after the patterning of the absorber layer 25 and after repairs that possibly have to be carried out thereon, the upper buffer layer 20 is removed in order to uncover the covering layer 15. As shown in FIG. 2b, for imaging purposes, EUV light is then radiated onto the upper surface of this EUV mask at an angle of typically 50 ° with respect to the normal to the surface, which light is absorbed (light beam LB) on the non-reflection regions NR covered with chromium, and is reflected by the multilayer coating 10 in the reflection regions R not covered by chromium and interferes constructively in the process (light beam LB′).
A series of technical problems result on account of the abovementioned multilayer arrangement of this known EUV reflection mask, in particular the arrangement of the absorber layer 25 as topmost layer above the reflective multilayers.
Thus, during the patterning and possible repair of the absorber layer 25, the underlying multilayers have to be protected by the upper buffer layer 20 since otherwise the periodicity of the interfaces of the multilayer coating 10 might be lost. The distance between the absorber layer 25 and the multilayer coating 10 is increased by the upper buffer layer 20 that is required. This results in the geometrical effects during imaging such as e.g. shading effects and line offset.
Since it is necessary to pattern not only the absorber layer 25 but also the underlying buffer layer 20, an inspection and repair qualification of both layers likewise becomes necessary. Overall, the entire mask system is highly susceptible to defects in the individual layers due to the relatively complicated multilayer construction.